In a Long Term Evolution-A (LTE-A) system, MIMO transmission of up to the Rank of 8 is supported, and at present the MIMO transmission scheme is implemented based upon a user equipment-specific demodulation reference signal, where indication information is transmitted together with data from the network side to a user equipment over a physical downlink control channel to indicate a user equipment-specific demodulation reference signal allocated for the user equipment, and the user equipment performs channel estimation on a corresponding antenna port allocated for the user equipment according to the indicated user equipment-specific demodulation reference signal so as to receive the transmitted data. Particularly the user equipment-specific demodulation reference signal to be used for the user equipment is indicated by indicating a demodulation reference signal pattern, an antenna port and a scrambling identity of a reference signal sequence allocated for the user equipment.
In an existing standard, there are four demodulation reference signal patterns allocated for a user equipment, i.e., the Rank-2 pattern, the first Rank-4 pattern, the second Rank-4 pattern and the Rank-8 pattern, which correspondingly support at most two antenna ports, four antenna ports, four antenna ports and eight antenna ports allocated for the user equipment respectively, where the number of allocated antenna ports is the value of a rank.
FIG. 1 illustrates a schematic diagram of the Rank-2 pattern, where the horizontal axis represents time and the vertical axis represents frequency, and each small square in the figure represents a Resource Element (RE), a square shadowed with inclined strips is a location where a common reference signal is located, and a square shadowed with horizontal strips is a location where a user equipment-specific demodulation reference signal is located; and among twelve squares REs shadowed with horizontal strips in the Rank-2 pattern, every two REs, adjacent in the time domain, of the same frequency subcarrier constitute a group, and are distinguished by an orthogonal code OCC length of 2 in combination with an indicated reference signal sequence, that is, allocation of at most two antenna ports for a user equipment can be supported.
FIG. 2 a schematic diagram of the first Rank-4 pattern, where a square shadowed with vertical strips is a location, where an added user equipment-specific demodulation reference signal relative to the Rank-2 pattern is located; and among twenty-four squares REs shadowed with horizontal strips and squares REs shadowed with vertical strips in the first Rank-4 pattern, every two REs, adjacent in the time domain, of the same frequency subcarrier constitute a group, and are distinguished by an OCC with a length of 2 in combination with an indicated reference signal sequence, that is, allocation of at most four antenna ports for a user equipment can be supported.
FIG. 3 a schematic diagram of the second Rank-4 pattern, where the number of user equipment-specific demodulation reference signals and their locations are the same as in the Rank-2 pattern illustrated in FIG. 1, but among twelve squares REs shadowed with horizontal strips in the second Rank-4 pattern, four REs, in the time domain, of the same frequency subcarrier constitute a group, and are distinguished by an OCC with a length of 4 in combination with an indicated reference signal sequence, that is, allocation of at most four antenna ports for a user equipment can be supported.
FIG. 4 a schematic diagram of the Rank-8 pattern, where the number of user equipment-specific demodulation reference signals and their locations are the same as in the first Rank-4 pattern illustrated in FIG. 2, but among twenty-four user equipment-specific demodulation reference signals in the Rank-8 pattern, four user equipment-specific demodulation reference signals, in the time domain, of the same frequency subcarrier constitute a group, and are distinguished by an OCC with a length of 4 in combination with an indicated reference signal sequence, that is, allocation of at most eight antenna ports for a user equipment can be supported.
Two reference signal sequences allocated for a user equipment are further defined in the existing standard and can be distinguished and identified by two different scrambling identities.
Based upon the forgoing existing standard, when a user equipment-specific demodulation reference signal is indicated, a user equipment has to be notified from the network side of which demodulation reference signal pattern of the foregoing three or four demodulation reference signal patterns is a demodulation reference signal pattern allocated for the user equipment, an antenna port allocated for the user equipment and which scrambling identity of the two scrambling identities is a scrambling identity of a reference signal sequence allocated for the user equipment.
Thus if a demodulation reference signal pattern, an antenna port and a scrambling identity of a reference signal sequence allocated for a user equipment could be arbitrarily selected and combined when a user equipment-specific demodulation reference signal is indicated, then it will means a large number of allocation schemes, thus resulting in a considerable overhead of signaling for indicating the user equipment-specific demodulation reference signal and wasting resources of a system.